This invention relates to a semiconductor device using a SOI (Silicon On Insulator) substrate, and more particularly to a device structure in which this feature is employed in the silicon layer portion of the semiconductor device.
In a SOI substrate, a silicon layer is formed on a layer with insulation properties, a so-called BOX oxide layer. This silicon layer is isolated by a trench structure or a LOCOS (Local Oxidation of Silicon) method in order to isolate the device. The trench method in which a groove is formed by the silicon layer being etched and an oxide layer being deposited in the groove is disclosed in "IEEE ELECTRON DEVICE LETTERS, VOL. 6, JUNE, 1995," and others. The cost of isolation using the trench structure is high because the number of processes required for trench structure formation is greater than the LOCOS method.
The device isolation for SOI by the LOCOS method is disclosed in "Proceedings IEEE Intr. SOI conf., 116 (1995)." According to the LOCOS method, a thin silicon layer the sectional shape of which is a triangle is formed between a LOCOS oxide layer and a BOX oxide layer and this layer forms the parasite MOSFET. This parasite MOSFET influences significantly the current property of the original (on the assumption that there is no MOSFET) MOSFET. This influence is called a bump property because it looks like a bump is made on the current properties. The threshold voltage for which the parasite MOSFET exists is lower than the original MOSFET.